1. Field of the Invention
The present invention relates to a device for inspecting an individual frequency signal selected according to frequency from a broad frequency band comprising a voltage controlled oscillator which can be controlled in terms of frequency by a periodically repeating sweep voltage in such a manner that all signals occurring in the frequency band can be displayed on a spectrum basis on the picture screen of a cathode ray tube, and further comprising a frequency mark generator which comprises a device for generating a frequency mark voltage and a voltage comparator which, given sweep operation and equality between and adjusted frequency mark voltage and a momentary sweep voltage, a frequency mark can be generated on the picture screen, the frequency mark voltage providing tuning for an oscillator to the corresponding fixed frequency value after termination of the sweep operation for the purpose of inspecting an individual signal selected by the frequency mark.
2. Description of the Prior Art
A device of the type set forth above is known in the art, for example from the German Letters Patent No. 1,238,081. This device makes it possible, on the one hand, for a frequency analysis in which all signals occurring in a specific, wide frequency band are displayed, according to spectrum, on the display of a cathode ray tube, the display being provided by means of periodic sweep of the frequency band with a sweeping oscillator which is controlled in terms of its frequency course by means of a periodically repeating sweep voltage, for example, a linearly increasing sawtooth voltage. By means of a transfer, a time analysis is also possible with this device, given which time analysis an individual high frequency signal selected according to frequency from the frequency spectrum displayed in the frequency analysis is further inspected. The selection of the desired individual signal occurs by means of a frequency mark which is displaceable along the spectrum by an adjustment device and which can be brought into coincidence with the selected individual signal, namely a reference voltage is generated in the adjustment device and a keying pulse is generated by the reference voltage, the keying pulse being supplied to the display system in such a manner that the adjustable frequency mark generated by means of the keying pulse, for example, an intensification pip, is brought into coincidence with the selected individual signal of the spectrum to be inspected. The reference voltage of the adjustment device determined in that manner and corresponding to the frequency value of the individual signal to be inspected is supplied to the sweep oscillator after termination of the periodically repeating voltage course and the sweeping oscillator is thereby matched to the corresponding fixed frequency value of the selected individual signal.
In high frequency receiving devices, more and more synthesizers are being employed recently for frequency matching, for example, phase-control oscillators operating according to the analysis, or, respectively, indirect frequency synthesis method, in which oscillators a voltage-control, free-running oscillator is controlled by way of a phase control loop in which the output frequency of the oscillator, mixed or divided down, for example, by means of an adjustable frequency divider, is compared with a highly-constant reference frequency in a phase comparison element. By switching off the phase control loop, this free-running oscillator can be controlled in frequency by means of a corresponding drive with a sweep voltage and can thus be exploited as a sweeping oscillator. Given receivers equipped with such phase-locked loop synthesizers, there is no adjustment device which also emits a d.c. voltage corresponding to one of the momentary fixed frequency settings of the synthesizer during the sweep operation, as is the case, for example, of a potentiometer provided in the known devices set forth above, by means of whose voltage, on the one hand, the frequency marks are matched along the spectrum and, after shutdown of the voltage-control oscillator, are matched to the selected fixed frequency. In such synthesizers, given a closed phase control loop, a matching d.c. voltage for the oscillator is available in this loop, the matching voltage corresponding to the respectively set fixed frequency of the synthesizer; this voltage corresponds to the frequency adjustment, however, but is not available when the phase control loop is interrupted during the sweep operation, during which the oscillator is directly driven by way of a sawtooth voltage.